Previously undefined immuno- and bioactive estrogens have been found in the urine of pregnancy in diverse primate species. These substances not only reflect fetal well-being but allow for the anticipation of parturition when they are the major excretory components. Although these estrogenic substances have not been completely characterized, most known estrogen molecules can confidently be . A major problem in conducting fetal endocrine research which is relevant to human investigations is the availability of an appropriate model. Hominoids differ from most other mamalian species in the amount of estrogen produced by the feto-placental unit and none of the laboratory or domestic animal models share this trait. In hominoids, the major estrogenic metabolite is estriol as a direct result of placental aromatization of fetal adrenal dehydro-epiandrosterone after fetal hepatic 16-hydroxylation and this metabolic pathway has not been found in any other animal species. Based on the hypothesis that fundamental physiologic traits are conserved across phylogeny, estrogens common to all primate pregnancies should reflect a basic physiologic mechanism in the human pregnancy. Conversely, the recognition of previously undefined urinary estrogens during pregnancy of lower primates partially explains the enigmatic production of estriol in pregnancy of only th hominoids. The present proposal is directed toward the elucidation of the phylogenetic distribution of these substances, their structural identification and physiologic significance.